Answer: This can only be determined by the very great Kabbalists, who see the world in the second half of the spiritual ladder.
They seemingly draw a person, absorb him into themselves, and then feel what his capacity, the unfilled volume of his soul is.
A Kabbalist who is only a beginner, however, cannot determine anything. To do this, you need to have heavy vessels of reception.
From KabTV’s “Fundamentals of Kabbalah” 11/25/18